ME Bank internet and mobile banking went offline on Saturday for ten hours thanks to an unplanned outage that also took down the bank's call centre.
ME Bank first notified customers of the "completely unplanned" technical issues at around 11am AEST. It said it hoped to have systems restored by 4pm AEST, but it wasn't until 9:45pm AEST that it reported systems had been restored.
The outage also shuttered phone services, leaving the bank's customer service team left to resort to its Facebook and Twitter accounts to respond to angry customers and report on the status of the rectification efforts.
The bank said the outage was "completely unforeseen" and not related to any scheduled work.
It did not respond for request for detail on the cause.
Customer cards for ATM and payment transactions were unaffected.
The bank said it would "do what we can" to reverse fees incurred by customers as a result of the unplanned outage.
"We know this is frustrating, and we're sorry for the inconvenience while we work to restore access," ME Bank said on social media.